M-Broth

Principle And Interpretation

Salmonella are facultatively anaerobic gram-negative bacilli that are typically oxidase negative, lactose negative, H2S positive and produce gas. Salmonella are found in nature and occur in the intestinal tract of many animals, both wild and domestic. Salmonella species cause variety of human diseases called Salmonellosis, which includes diarrheal disease and bacteremic conditions leading to enteric fever. The severity of the diarrheal diseases depends on the virulence of the strain and the condition of the human host. Serological procedures that confirm the identification of an organism are usually agglutination reactions.

M-Broth was developed by Sperber and Diebel (6) to accelerate the detection of Salmonellae. The accelerated 50 hour detection procedure consists of an 18 hours pre-enrichment, a 24 hours selective enrichment, a 6 to 8 hours selective enrichment and 2 hours serological testing. In the selective enrichment step, to avoid nonspecific agglutination, Sperber and Diebel modified APT Broth by removing dextrose from it and by adding mannose. Fantasia et al (2) found that enrichment serology method is rapid and less complicated to perform than the method described in the Bacteriological Analytical Manual (1) by maintaining the accuracy and sensitivity of the method. M-Broth also conforms to testing standards recommended by APHA (5) for isolation and identification of foodborne Salmonella. M-Broth contains all the nutrients necessary for good growth and flagella development of Salmonella.

Tryptone and yeast extract in the medium provide organic nitrogen, carbon, sulphur, vitamins and trace elements essential for the growth of Salmonella species. Mannose is the fermentable sugar and energy source and it prevents fimbrial agglutination of Salmonella (6). Sodium chloride helps to maintain osmotic equilibrium. Dipotassium hydrogen phosphate acts as a buffer. The inorganic salts stimulate bacterial growth while polysorbate 80 supplies fatty acids.

10% suspension of sample is prepared in sterile Lactose Broth (M026) and incubated at 35±2°C for 18 to 24 hours. 1 ml of this pre-enriched culture is added to 9 ml of Selenite Cystine Broth (M025) and Tetrathionate Broth (M032) and incubated at 35±2°C for 24 hours. This enriched culture is subsequently inoculated in M-Broth and incubated at 35±2°C for 6-8 hours following, which H-agglutination test is performed as per standard procedures.